The use of hydrogen peroxide as an agent for the oxidative treatment of waste water takes on increasing significance under today's aspects of environmental protection, since hydrogen peroxide itself does not cause additional salting of the waste water but splits into water and oxygen. However, an essential condition is that the necessary amount of hydrogen peroxide can be determined automatically and controlled.
This requirement is especially true for the treatment of continuous waste water streams which contain variable amounts of different oxidizable toxic materials and only permit a relatively short time for treatment. In such cases the success of waste water treatment with hydrogen peroxide depends highly on how quickly and reliably the total requirements of oxidizing agent can be determined in the incoming waste water stream in the treatment plant. As total oxidizing agent demand (requirement) we understand that amount of active oxygen or H.sub.2 O.sub.2 which is needed in order to be able to convert the oxidizable material in the waste water under the chosen reaction conditions into the appropriate oxidation products in each case.
Little information can be found in the technical literature as to how the total oxidizing agent requirement of a waste water can be determined automatically. The reason for this is that most of the materials occurring in waste water only contribute in small and very variable manner to the formation of a definite redox potential, that e.g. can be measured with a platinum/reference electrode pair. Therefore the oxidative waste water treatment process is operated in such a way that, while following the redox potential of the waste water, oxidizing agent is added successively until due to the presence of an excess a constant end point is established. However, in most cases this process is usable only for a batchwise treatment of waste water, since the oxidation of the various materials with H.sub.2 O.sub.2 requires variable reaction times and therefore a constant end potential is not established immediately, even in the presence of excess H.sub.2 O.sub.2, but only when the oxidation reactions are completed.
Thus, e.g., there is described in German Pat. No. 2,352,856 and related Fischer U.S. Pat. No. 3,970,554 a process for the detoxification of cyanide-containing waste water with H.sub.2 O.sub.2 that permits the monitoring and control of the detoxification process and the addition of the H.sub.2 O.sub.2 by measuring the redox potential with a silver electrode and any desired reference electrode, preferably a thalamide electrode. (The entire disclosure of the Fischer U.S. patent is hereby incorporated by reference and relied upon). However, this process is only employable in most cases for a batchwise treatment of waste water since the measurement of the redox potential shows the free cyanide content in the waste water, but permits no conclusions regarding the total oxidizing agent requirement if, besides free cyanides, other consumers of hydrogen peroxide are also contained in the waste water.
The single addition of an amount of oxidizing agent corresponding to the indication of free cyanide content at the beginning of the waste water treatment would accordingly lead to insufficient detoxification results, because the other consumers of H.sub.2 O.sub.2 use up a portion of the hydrogen peroxide which is needed for the cyanide oxidation. Only through multiple subsequent dosing of oxidizing agent at fixed time intervals can in these cases bring about both the desired breakdown of cyanide as well as of other oxidizable materials present. Such a stepwise approach to the goal of the process, as necessary because of the reaction time needed for the oxidation of the cyanide, does not usually permit the continuous treatment of large waste water streams because the treatment plant must be constructed so large in order to guarantee sufficient treatment time.
In Belgian Pat. No. 883,046 there is given a process for the measurement of the concentration of a specific dissolved component with redox- or ion-sensitive electrodes for the purpose of monitoring or regulating chemical processes, among others in the treatment of waste water. This process is characterized by the determination of the maximum attainable potential value by addition of excess reagent to a side stream of the system and evaluating the deviation of the starting potential as a measure of the reactive material content. For example, in this way the free cyanide content can be ascertained quickly and reliably in a continuous waste water stream and the amount of hydrogen peroxide shown to be required for the oxidation can be added. However, the process described according to Belgian Pat. No. 883,046 fails to work if there are present in waste water different oxidizable toxic materials in varying concentrations together which contribute different amounts to the formation of the redox potential. Thus, e.g., in the measurement of the cyanide potential with a silver electrode and a thalamide reference electrode under the conditions of the process described in German Pat. No. 2,352,856 (and related Fischer U.S. patent) approximately the same redox potential of about 350 mV is produced by 1 mg 5.sup.2-1 l as by 50 mg Cn/l. Other oxidizable materials, such as sulfite, on the other hand are practically not detected at all by this redox system.
Therefore the invention is based on the problem of developing automatic system which makes possible quick and reliable determination of the oxidizing agent requirement which is needed for decreasing the toxic material concentration of a waste water under a desired limit in a continuous oxidative treatment of the waste water under fixed pH and temperature conditions.